Take a look at this thread.
Brings up reticle on key press, fires @cursor on key release.
https://us.forums.blizzard.com/en/wow/t/on-release-macro/813974
If you would prefer two separate buttons, do this:
Macro 1
#showtooltip
/use Ground Target Spell
Macro 2
#showtooltip
/stopspelltarget
/use [@cursor]Ground Target Spell